A former assistant under Bill Belichick, Eric Mangini arrived in New York last year with an insider's knowledge of the Patriots' sign-stealing surveillance tactics and he shared the secret with members of the Jets' organization, a person with knowledge of the matter informed the Daily News yesterday. It wasn't until the fifth Mangini-Belichick showdown - last Sunday - that the Jets were able to catch the Patriots. Tipped off by Jets security, an NFL security official confiscated a video camera and tape from a Patriots employee at the Meadowlands, and the evidence is believed to be incriminating. "(The Jets) knew they did it," the person with knowledge of the situation said in an e-mail to the Daily News. "They caught the guy a year ago, but couldn't do anything about it. When Eric came, he said that's what they used to do. Bill is going to be (ticked) at Eric. He kissed and told."